Switch mode power supply (SMPS) overcomes the limitations of regulated power supplies.Here in SMPS (or sometimes called switching regulator), the pass transistor is used as a controlled switch and is operated either in cutoff region or saturation region which makes the power transfer via the pass transistor in the form of discrete.
As in the case with other digital switching devices,this mode of operation results is very low power consumption.
SMPS are used in modern digital equipment such as telephone exchange,PCs,robotics etc.,battery powered equipment,video projectors,measuring instruments,space vehicles etc.Such system need very compact,light weight and highly energy efficient supplies. SMPS utilizes pulse width modulation to control the average value of output voltage.The average value of the waveform depends on the area of waveform.If the TON time(or say,duty cycle (TON/TON+TOFF) is varied,the average value of voltage changes proportionately.
In the PWM switching at constant switching frequency,the switch control signal is generated by comparing Vcontrol and repetitive waveform .
SMPS or switching regulators can provide large load currents at low voltage precisely what is required in PCs (personal computers).
Switching regulators are available in three basic configuration viz. step-down,step-up and polarity inverting configurations.
Step-down version is shown in fig.The rectangular pulses on the base saturate and cut off the pass transistor during each cycle.This generates a rectangular voltage at the input to LC filter.This filter blocks the ac components and allows the dc components to pass to the output.Because of the on-off switching,the average value is always less than the input voltage.This is why the circuit is called the steep-down version.
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